An optical recording medium such as a CD with a recording capacity of 0.65 GB and a DVD with a recording capacity of 4.7 GB as a medium for storing video information, audio information, or data on a computer is spreading among people. Then, recently and continuing, there is rising demand for further increasing the recording density and the capacity.
As a method for increasing the recording density of such an optical recording medium, it is effective to increase the numerical aperture (referred to as NA, below) of an objective lens or to shorten the wavelength for a light source in an optical pick-up for performing writing or calling of information for the optical recording medium, so that the diameter of a beam spot focused by the objective lens decreases which spot is formed on the optical recording medium. Accordingly, for example, while the NA of the objective lens and the wavelength for the light source are defined as 0.50 and 780 nm, respectively, for a “CD-type optical recording medium”, the NA of the objective lens and the wavelength for the light source are defined as 0.65 and 660 nm, respectively, for a “DVD-type optical recording medium” having recording density higher than that of the “CD-type optical recording medium”. Then, further increase of the recording density and capacity of the optical recording medium is desired as described above, and therefore, it is desired to make the NA of the objective lens be greater than 0.65 or to make the wavelength of the light source be less than 660 nm.
For such a high-capacity optical recording medium and an optical information processing apparatus, two specifications are proposed. One of them is a “Blue-Ray Disc” specification which satisfies the ensuring of a capacity of approximately 22 GB using a source of light in a blue wavelength region and an objective lens with an NA of 0.85 as disclosed in Masuo Oku et al., “The goal of Blue-ray Disc”, Nikkei Electronics, pp. 135-150 (2003.03.31). The other is a “HD DVD” specification which satisfies the ensuring of a capacity of approximately 20 GB using a similar blue wavelength region and an objective lens with an NA of 0.65, as disclosed in Naoshi Yamada et al., “a next-generation specification derived from a DVD, “HD DVD””, Nikkei Electronics, pp. 125-134 (2003.10.13).
The former attains the capacity increase due to modifications such as the employment of wavelength shorter than that of DVD-type and a higher NA and the latter attains the capacity increase due to ingenious signal processing to allow the increase of linear recording density and the employment of land-groove recording to reduce a track pitch, instead of the employment of a higher NA.
As described above, the two specifications using the source of light within a blue wavelength region are proposed. However, it is desirable for a user to treat the optical recording media in accordance with the two specifications without distinction in a single optical information processing apparatus. As the simplest method for realizing it, there is provided a method of mounting plural optical pick-ups. However, it is difficult for this method to achieve miniaturization and reduced cost. Therefore, an optical pick-up that can perform recording or reproducing with a common light source and a common objective lens for the two blue-specifications is desired. However, the generation of aberration caused by tilt (inclination) of the optical recording medium or a thickness error of a transparent substrate is known as a problem of such an optical pick-up. The problem is such that the margins of the thickness error and the tilt are lowered in the case of shortening the wavelength for a light source and increasing the numerical aperture of an objective, that is, attaining higher density due to the reduction of the spot diameter of light focused on the optical recording medium.
First, spherical aberration caused by a thickness error of a transparent substrate of the optical recording medium is described. As the spherical aberration occurs, a spot formed on an information-recording surface of the optical recording medium deteriorates and, therefore, a normal operation for recording or reproducing cannot be carried out. Generally, spherical aberration caused by a thickness error of a transparent substrate of an optical recording medium is given by the following mathematical formula 1,
      W    ⁢                  ⁢    40    ⁢                  ⁢    rms    ≈            1              48        ⁢                  5                      ·                            n          2                -        1                    n        3              ·          NA      4        ·                  Δ        ⁢                                  ⁢        t            λ      wherein λ is a used wavelength, NA is the numerical aperture of an objective lens, n is an equivalent refractive index of the optical recording medium, and Δt is a shift along the direction of the optical axis from a spot position at which the spherical aberration is the minimum.
FIG. 1A shows respective aberration quantities caused by a substrate thickness error of the first optical recording medium with an NA of 0.85, a substrate thickness of 0.1 mm, and a used wavelength of 405 nm. In the figure, SA is spherical aberration, COMA is coma aberration, TOTAL is total amount of these secondary aberrations, and STREHL is the peak intensity of a spot. FIG. 1B shows respective aberration quantities caused by a substrate thickness error of the second optical recording medium with an NA of 0.65, a substrate thickness of 0.6 mm, and a used wavelength of 405 nm.
Commonly, the tolerance of W40 rms is necessarily half or less of approximately 0.07 λrms, since it is empirically known that the quantity of wavefront aberration is necessarily smaller than Marechal's criterion (0.07 λrms) in reading a signal from an optical recording medium and it is necessary to also include aberration of an objective lens and aberration caused by tilt of the optical recording-medium when the wavefront aberration is considered. Also, for a substrate thickness error of an optical recording medium, a molding tolerance of approximately ±10 μm is necessarily considered; therefore, compensation is needed for the first optical recording medium.
Second, coma aberration caused by tilt (inclination) of an optical recording medium is described. As the coma aberration occurs, a spot formed on an information-recording surface of the optical recording medium deteriorates and, therefore, a normal operation for recording or reproducing cannot be carried out. Generally, coma aberration caused by tilt of an optical recording medium is given by the following mathematical formula 2,
      W    ⁢                  ⁢    31    ⁢                  ⁢    rms    ≈                              n          2                -        1                    2        ⁢                  n          3                      ·    d    ·          NA      3        ·          θ      λ      wherein n is the refractive index of a transparent substrate of the optical recording medium, d is the thickness of the transparent substrate, NA is the numerical aperture of an objective lens, λ is the wavelength for a light source, and θ is a tilt quantity of the optical recording medium.
FIG. 2A shows respective aberration quantities caused by tilting of the first optical recording medium in an optical system with an NA of 0.85, a substrate thickness of 0.1 mm, and a used wavelength of 405 nm. Similarly, FIG. 2B shows respective aberration quantities caused by tilting of the second optical recording medium in an optical system with an NA of 0.65, a substrate thickness of 0.6 mm, and a used wavelength of 405 nm.
Commonly, the tolerance of W40 rms is necessarily half or less of approximately 0.07 λrms, since it is empirically known that the quantity of wavefront aberration is necessarily smaller than Marechal's criterion (0.07 λrms) in reading a signal from an optical recording medium and it is necessary to also include aberration of an objective lens and aberration caused by a thickness error of the optical recording medium when the wavefront aberration is considered. Also, for the tolerance of manufacturing the optical recording medium and the precision of mounting the optical recording medium, the occurrence of tilt of approximately 0.3 degrees is necessarily considered; therefore, compensation is needed for the second optical recording medium.
As described above, it is necessary to compensate for the spherical aberration caused by a substrate thickness error of the first optical recording medium and the coma aberration caused by tilting of the second optical recording medium in an optical pick-up to perform recording or reproducing for both the first optical recording medium and the second optical recording medium. For the compensation, it is necessary to detect and dynamically compensate for the quantity of generated aberrations, but the simultaneous compensation of the coma aberration and the spherical aberration requires complex control. Also, to provide both a coma aberration compensation element and a spherical aberration compensation element causes an increase of both the number of components and the size of the optical pick-up.